This invention relates to a high efficiency encoding device and a noise spectrum modifying device and method for speech or audio signals. More particularly, it relates to a high efficiency encoding device and a noise spectrum modifying device and method of the sub-sampling encoding system, such as MUSE, in which the quantization noise spectrum is rendered aurally acceptable by reducing the noise level as perceived by the ear to improve the quality of the speech or audio signals for high definition telecasting.
As a system for high efficiency encoding of digital audio signals there has hitherto been proposed a system consisting in the combination of differential encoding and near-instantaneous compression. According to the proposed system a difference between two neighboring samples of the digital audio signals is compressed on a block unit basis, by way of near-instantaneous compression, using range information as found based on a maximum value of the difference in a block consisting of a preset number of samples.
There has also been disclosed in JP Patent KOKAI Publication No. 63-16718 (1988) a technique in which the difference is found based on a difference between signals of the current sample and signals of a directly previous sample restored by processing with near-instantaneous compression, near-instantaneous expansion and accumulation to correct the errors produced at the time of the range compression,
Such high efficiency encoding system, consisting in the combination of the differential encoding and near-instantaneous compression and expansion (compansion) is employed for encoding audio signals processed with bandwidth compression by the multiple subsampling encoding system, such as MUSE, for telecasting or recording on a recording medium, such as a recording disc, of high definition television signals, such as so-called high-vision signals.
FIG. 1 shows characteristics in the high-efficiency encoding by the above-described conventional combination of the differential encoding and near-instantaneous compansion. In FIG. 1, curves A and B represent an output signal level and a noise level, respectively. Thus it is seen that the S/N ratio is lowered towards higher frequencies at a rate of 6 dB/oct. The quantization noise shows a substantially flat spectrum. The status shown in FIG. 1 is not fully satisfactory as long as the aural sense of human being is concerned, although a minimum noise energy may thereby be achieved.
Above all, the S/N ratio is lowered when the high-range signal is supplied as an input signal.